Hatsukoi Aite no Kimi wa Dare? manga launched on March 8
The manga launched in Weekly Manga Times on March 8. The story of the spinoff follows high-school student Takeru who, while in elementary school, became interested in his classmate Marika, the reincarnated wife of the series, who has matured recently.
Noriyuki Abe (Yū Yū Hakusho, Bleach, Flame of Recca) is directing the anime at St.Signpost. Sawako Hirabayashi (Wolf Girl & Black Prince, Delicious Party♡Precure) is supervising and writing the series scripts. Narihito Sekikawa is designing the characters.
The manga centers on Keisuke Niijima, a man who remains brokenhearted 10 years after his wife Takae passed away. But one day, an elementary school girl comes to his house and says that she is Takae, reincarnated as a different girl. Now Keisuke is trying to make up for lost time, while Takae struggles as a grade schooler with the mind of a woman in her 40s, trying create situations where she can meet Keisuke.
Murata published a one-shot version of the manga in Weekly Manga Times in April 2018, before beginning serialization of it in July that same year. The manga ended in December 2022, and Hobunsha published the manga's 14th and final compiled book volume in March 2023.
Update: Corrected date of manga's debut. Thanks SUPER7X.
The manga already inspired a live-action series that aired from January to March 2022.
Source: Weekly Manga Times issue 15